Field of the Invention
The present invention relates to an AC-DC converter which is improved to increase the power factor and to reduce generation of higher harmonics of a commercial power line.
A rectifier/smoothing circuit of capacitor-input type has been used as means for rectifying an A.C. input of an AC-DC converter circuit of various types of electronic apparatus, by means of its simple construction. In recent years, however, a problem has been reported in which higher harmonics are undesirably generated in commercial power line by electronic apparatus having a rectifier/smoothing circuit of capacitor-input type, resulting in erroneous operation of other electronic apparatus connected to the same power line.
This has given a rise to the demand for power-factor improved AC-DC converter in which generation of higher harmonics is suppressed. To cope with this demand, the AC-DC converters have been proposed which employs various measures for improving power factor.
FIG. 7 shows the circuit of a known AC-DC converter of improved power factor type. The AC-DC converter as shown in FIG. 7 has the following circuit construction. A.C. input terminals of a rectifier circuit 1 is connected to a commercial power line through input terminals 20a, 20b of the converter. Output terminals of the rectifier circuit 1 are connected to input terminals of an active filter 3, output terminals of which are connected to input terminals of a DC-DC converter 2. A detailed circuit construction of the DC-DC converter 2 and a load connected to its outputs are omitted.
The active filter 3 of the AC-DC converter as shown in FIG. 7 has the following circuit construction.
A choke coil L1 and a diode D1 are connected between a higher potential output terminal of the rectifier circuit 1 and a higher potential input terminl of the DC-DC converter 2. The forward direction of the diode D1 is determined such that current is allowed to flow from the choke coil L1 to the DC-DC converter 2.
A lower potential output terminal of the rectifier circuit 1 is connected to a lower potential input terminal of the DC-DC converter 2 and the ground. A series circuit of a principal current path of a switching transistor Q1 and a resistor R4 is connected between the anode of diode D1 and the lower potential output terminal of rectifier 1. An output capacitor C1 is connected between the cathode of diode D1 and the lower potential output terminal of rectifier 1, and a series circuit of resistors R1 and R2 is connected in parallel across the output capacitor C1.
A negative input terminal of a voltage error amplifier 5 is connected to a connecting point of the resistors R1 and R2. A positive input terminal of the voltage error amplifier 5 is connected to a reference voltage source 6, and its output terminal is connected to one of input terminals of a multiplier circuit 13.
Another input terminal of the multiplier circuit 13 is connected to the higher potential output terminal of the rectifier 1, an output terminal of which is connected to a negative input terminal of a comparator 12. A positive input terminal of the comparator 12 is connected to a connecting point of the switching transistor Q1 and a resistor R4, an output terminal of which is connected to a reset input terminal (R) of a flip-flop 10. A set input terminal (S) of a flip-flop 10 is connected to an output of an oscillator 7, and an output terminal (Q) of the flip-flop 10 is connected to a gate of the transistor Q1.
A following description will now be given of the operation under the foregoing circuit construction.
When a switching pulse outputted from the oscillator 7 starts to rise so that a voltage is inputted to the input terminal (S) of the flip-flop 10, a voltage of the output terminal (Q) of the flip-flop 10 rises up and the switching transistor Q1 turns on.
When the switching transistor Q1 is at ON state, current flows through choke coil L1, switching transistor Q1 and resistor R4. This current is in inverse proportion to inductance of the choke coil L1 and its value gradually increases in proportion to time that an voltage is applied.
This current value is detected as voltage value through the resistor R4 which is inputted to one of the input terminals of the comparator 12. When this detected voltage value is greater than a voltage value of a control signal from the multiplier circuit 13, a voltge at the output terminal of the comparator 12 rises. The control signal from the multiplier circuit is determined by both an output voltage of the rectifier 1 and the output of the voltage error amplifier 5.
This output of the voltage error amplifier 5 is in proportion to the difference of a voltage which is obtained by dividing the voltage across the output capacitor by resistors R1 and R2 and the reference voltage of the reference voltage source 6.
When the voltage which has risen at an output of the comparator 12 is inputted to the reset input terminal (R) of the flip-flop 10, the voltage of the output terminal (Q) falls and the switching transistor Q1 turns on.
As described above, the conventional AC-DC converter can easily realize a high power factor. The converter, however, becomes complicated and increases in size because a multiplier circuit is needed as a circuit component.
In order that an oscillator is used for driving a circuit in both the active filter and the DC-DC converter, many circuit elements are needed and an apparatus become large in size. In addition to this, the AC-DC converter has disadvantage of increasing EMI by means of its oscillating pulse because there exist a plurality of oscillators therein.